When I tried to run ./configure --prefix=$HOME/emacs-xwidgets --with-x-toolkit=gtk3 --with-xwidgets I get an error configure: error: xwidgets requested but gtk3 not used.. But it looks like I have an gtk+3.0 -

Are you going to be using x11, or do you intend this to be a part of the standard GUI version on OSX without x11? See also my related question on reddit -- What is the macports equivalent for xwidgets?: reddit.com/r/emacs/comments/4bfef1/…
– lawlistAug 3 '16 at 5:57

Get the same error message here, after appending --without-ns, it now says configure: error: xwidgets requested but WebKitGTK+ not found. Though I have installed webkitgtk via homebrew.
– xuchunyangAug 3 '16 at 6:13